The function of this core unit is to assist investigators of this PPG in the in vivo functional evaluation of human cells transduced by their vectors. This will be achieved by using transplantations of transduced human hemopoietic cells into a receptive xenogeneic host (NOD/SCID or NOD/SCID/beta2/mug-/-). The unit will generate and maintain sufficient numbers of NOD/SCID or NOD/SCID/beta2/mug-/- mice, to accommodate the needs of investigators of this PPG, and will be responsible for the mechanics of transplantation (i.e., irradiation, i.v., or intramarrow injection of donor cells). In addition, the unit will undertake the sampling and assist, at least in part, in the post-transplant evaluation required to assess the fate of the transplanted human cells.